This invention relates in general to systems for inspecting surfaces and in particular, to a system for locating features of a surface.
Stylus profilometers and/or scanning probe microscopes have been used for inspecting features of surfaces, where such features may include the topography, dimensional information, electrical resistance or capacitance or magnetic characteristics, frictional properties, van der Waals forces, light emission or transmission behavior and other physical properties.
In order to locate one or more of such features, optical techniques using video cameras, objective lenses, and pattern recognition have conventionally been employed. These techniques are limited by the resolution of the optics and of the camera, the optical magnification power of the optics, the illumination angle and pattern recognition software algorithms. Even with a high power objective lens and using the most robust pattern recognition software available at present, an object can still fail to be located optically to a desired degree of accuracy. After the feature has been located optically, the stylus tip of the scanning probe microscope or of the profilometer is then moved to a desired location relative to the feature in order to measure the feature. If optical techniques are used to position the feature relative to the stylus tip, the stylus may hinder the path of view of the optics for precise positioning of the feature under the stylus. If the positioning of the feature is performed using a "coarse stage", the repeatability of the positioning of large stages, such as wafer or disk handling stages, is somewhat limited to about 0.25 microns so that the object cannot be positioned to an accuracy better than 0.25 microns.
Due to the uncertainties inherent in the positioning procedure described above, the user cannot be certain as to the position of the stylus tip relative to the feature to be measured. For this reason, in conventional methods, even after the stylus tip has been located roughly close to the feature to be measured, more accurate positioning still needs to be achieved. One must first designate a window or area of interest that contains the feature and acquire high resolution two-dimensional or three-dimensional data of the feature within such window or target area. The stylus tip and sensor are used to acquire high resolution two-dimensional or three-dimensional data in an area containing the feature of interest. The data is then examined for an indication of the feature of interest, and the data is used to position the stylus tip at the exact location of interest. It is only at such point that the user is then ready to use the stylus tip to measure the feature.
The above-described technique for positioning the stylus tip in position to measure the feature has several shortcomings. If the feature is not one that can be located optically, the above-described optical technique cannot be used. Even for features that can be optically located, the above-described procedure is time consuming and wasteful of resources. Due to the uncertainty inherent in the optical technique, conventional positioning techniques require the acquisition of the full image within a target area in the manner described above. For example, if a scanning probe microscope is used to acquire a 256.times.256 data point full image, such data acquisition typically requires four to five minutes. Most of these data points will be discarded after the feature is located. Therefore, the above-described process is limited in throughput and is wasteful of the user's resources. It is, therefore, desirable to provide an improved system for locating and then measuring a feature of interest, where the system can be used even when the feature is not optically detectable.